Unravelling the Crucial of Spatial Al Distribution to Realize Precise Alkali‐Treatment for Target Acid‐Catalyzed Reactions

Abstract

Constructing mesoporous structure within zeolites by alkali‐treatment is an effective protocol to improve their diffusion properties. However, undesirable changes in Brönsted acid site (BAS) densities always offset this advantage in acid‐catalyzed reactions. In this context, the crucial roles of spatial aluminum distribution were unraveled during alkali‐treatment of MFI zeolite and the desirable BAS density was achieved in obtained hierarchical samples for the target reactions. Various characterization methods, particularly the multiple one‐ and two‐dimensional magic‐angle‐spinning (MAS) NMR techniques, were performed to track the alkali‐treatment processes. For the sample with a more uniform spatial Al distribution, more tetrahedral Al sites would fall off and migrate around the Si‐OH in zeolite as Al(OH)4‐. Those re‐deposited Al(OH)4‐ sites were easily transformed into NMR‐invisible Al sites during the calcination process, which contributed negligibly to both Brönsted and Lewis acidities, thus being referred to“acid‐free”Al species. While most tetrahedral Al sites were preserved after the alkali‐treatment of sample with non‐uniform Al distribution and the BAS density gradually increased with treatment time. According to the requirements of typical acid‐catalyzed reactions, such as catalytic cracking of 1,3,5‐triisopropylbenzene and methanol‐to‐olefins, the desired hierarchical zeolite catalysts were developed by matching the amounts of extracted Si and generated“acid‐free”Al during the precise alkali‐treatment.